linux-ext4 - Re: [RESEND] [PATCH 2/2 v2] e2fsck: Do not forget to discard last block group

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Date:	Fri, 24 Feb 2012 11:48:32 -0500
From:	Phillip Susi <psusi@...ntu.com>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	linux-ext4@...r.kernel.org, tytso@....edu, sandeen@...hat.com
Subject: Re: [RESEND] [PATCH 2/2 v2] e2fsck: Do not forget to discard last
 block group

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On 2/24/2012 11:13 AM, Lukas Czerner wrote:
> It does not work that way, uinit is never set back. If it has been 
> formated without discard it is user choice, moving the image to
> the thinly provisioned device, or ssd with dd is really bad idea
> anyway. That said, UNINIT means that it has not been used and hence
> there should be nothing to reclaim.

I could have sworn that e2fsck set it back when the block group became
free again, since there is once again no need to parse the bitmap and
you can just assume it's empty without having to read it.  I certainly
have e2defrag doing this.  If fsck and the kernel currently don't do
this, they should.

Whether it is a bad idea or not, people do move filesystems around and
have existing systems formatted before mke2fs would issue discards, so
it is a good idea to discard unused areas regardless of whether or not
they are uninitialized.

My understanding of uninitialized is that it was added as an
optimization meaning "there's nothing here, so you can skip/ignore
this" rather than "this has _never_ been used, so you can rely on it
containing all zeros and being discarded".

Indeed, a quick test filling a block device with random data and
running mke2fs on it leaves the random data in the uninitialized block
bitmaps rather than writing all zeros.

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